L233
Cooperative Extension Service
The University Of Georgia I College of Agricultural
and Environmental Sciences /Athens
Upper Layer
by
George W. Lewis
Aquaculture Coordinator
From June to October, oxygen depletion is the most
common cause of fish kills in Georgia ponds. Depletion
of oxygen In pond water will occur when conditions
create a greater demand for oxygen than the environment can produce. When ftsh are observed dying because of oxygen depletion, it is often too late to stop the
kill. However, oxygen depletion can often be anticipated
and detected before fish begin to die and measures can
be taken to prevent a fish kill.
Figure 1.
Oxygen depletion in a pond is the result of demand
exceeding supply. Aquatic animals, plants, and decaying organic matter consume oxygen. Aquatic plants are
primary producers of oxygen. Plants produce oxygen as
a by-product of photosynthesis. The rate of photosynthesis IS dependent upon light. Because of the effect of
sunltght on photosynthesls, the amount of oxygen in
pond water fluctuates daily. Oxygen levels are usually
htghest at midday and lowest just before sunrise. Fish
klils usually occur when more oxygen IS consumed during the night hours than is produced during daylight
hours. Warm water does not contain as much oxygen
as cold water. Thus. during warm weather months, mismanagemei;t of ponds by overstocking. overfeeding.
overfertillzatlon, pollution from barns and feedlots. 01
chemical treatment of aquatic weeds can result In oxygen depletion and fish kills.
During warm weather months. pond water stratifies
into three layers (Figure 1). The top layer is warmer and
contains oxygen producing algae. Wind action also contributes oxygen to the top layer. Most of the fish are in
the top layer. The middle layer is a relatively thin layer
characterized by a rapid drop in temperature and oxygen levels The bottom layer is stagnarlt. cool water
with little or no oxygen. It does not mix with the upper
level during warm weather. Fish rarely enter this area.
Oxygen depletion can occur when these layers mlr
during a “turnover.” Fish kills will occur if oxygen demand in the stagnant bottom layer of water exceed? the
oxygen available in the top layer. Turqo\/er ra- bp
triggered by heavy rains, strong winds b!owing fry lono
periods in one direction, and rapid cooling i” the fa!’
Several calm, cloudy days during wa;m w?;):%r car
also result in oxygen depletion and fish kiils T’-IIIC IS hecause the rate of photosynthesis and subsequent @xv
gen production is dependent on the intensity of l~ghi 1~
a pond with an excessive bloom of algae SPL+~~: d?vc.
of reduced light artd calm weat+r car TPRJI: I? r8?;:
enough oxygen being produced to meet thy &ma%+
of respiration and decay.
The pond owner should be alert to posstbliltv (I’ I?UV
gen depletion If one or more of the following condlr~\~*~~
exist:
(1) After a heavy rain:
(2) During periods of strong winds:
(3) During periods of calm. cloudy days:
(4) During the fall when air temperatures are raoldlv
cooling:
2
3
Causes of Oxygen Depletion
(5) After chemrcal treatment of aquatic weeds.
If one of these conditions exist, a pond owner should
inspect his pond daily for signs of oxygen depletion.
8.0
7.0
Signs of an Oxygen Depletion
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There are several signs of oxygen depletion:
(1) Large numbers of fish swimming to the top and gulprng air at night or early in the morning. If disturbed
they dive but quickly return to the surface.
(2) If oxygen depletion has not reached a lethal level,
fish are at the surface in the early morning but return
to deeper water as oxygen builds up during the day.
This may continue for several days. The pond owner
should take corrective action immediately.
(3) Fish being fed suddenly stop eating.
Testing the oxygen level in a pond is an excellent way
to detect a developing problem. Oxygen test kits and
meters are available for this purpose. Tests of oxygen
levels in ponds should be made at dawn or shortly thereafter. If oxygen is less than three parts per million (ppm)
in the top three feet of water, immediate action should
be taken.
Thus may be impractical for a farmer with a large
number of ponds. One can estimate the possibility of an
oxygen depletion by measuring oxygen in the earl; evening and agatn two or three hours later. Since the rate
of oxygen depletion IS somewhat linear. these two
values can be used to extrapola?e oxygen content versus time durtng the remaining hours of darkness. Figure
2 rllustrates this method.
The needs of fish for oxygen vary according to
species. age and culture conditions. Most warm water
fish need oxygen dissolved in water at a rate of at least
one ppm for survival and more than three ppm for comfort Oxygen at we or more ppm provides the best growxng conditions f’
Emergency Treatments
If srgns of oxygen depletion are observed, emergency
treatment must be undertaken immediately to prevent
loss The followrng physical and chemical emergency
treatments are effective.
Physical Treatments:
The most effective emergency treatment is mechanical aeration of the water. There are a number of ways
the water can be aerated mechanically Whichever
method is used, the sooner it is applied and the larger
the volume of water sprayed or agitated per unit time
and the sooner a current is established the more effective the method will be. If mechanical aeratron !s used
it is important not to disturb the bottom mud. Bottom
mud contains a large amount of organic material ant
decomposing bacteria that will contribute to oxygen depletion problems if mixed with water.
1. Paddle Wheel Aerator. There are various designs for
a paddle wheel aerator Essentially. it is constructec
by mounting a used car or truck differential on a trailer
frame, constructing a paddle wheel on each end of the
differential, and attaching a power-take-off linkage tc
the differential. The hub of the paddle whael can be
an 18-inch diameter metal cylinder. Paddle blades are
usually 8 to 12 inches long and the width of the drum
Figure 3 is an example of the paddle wheel design.
Ideally, the paddle wheel should be backed into a
pond until the bottom blades are just under the water
surface. The differential should be level with respect
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Projected Value .
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Figure 2.
Chemical Treatments:
clamp - do not weld
to housing
% ton differential
orequal
A = T’pipe
B = 3’ pipe
* % pillow block
bearing
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I_,
:p
-
6
? oioe
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04
a( _
’ 3’i’pipe
IV
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eta, /
I NOTE:
. paddles should
be level with
respect to water
surface.
2%
p 4’ pipe
Figure 3. POND PADDLEWHEEL AERATOR
to i?e water sutiace The paddle wheel IS efficiently
operated at i ,5GG to 1.800 rpm in large ponds; a reduced speed m&y be necessary in small ponds Paddle wheels should turn toward open water.
2 Large Volume Pumps. Any large volume water pump
can be used to aerate a pond. Ideally. the pump
should set up so that it creates a current and at the
same time blows or sprays water across the pond surface.
3. Bushhog Mower. Bushhog mowers can also be used
to aerate a pond. Essentially. they are backed into the
water and the mower blade is used to circulate the
water
4. Outboard Motors. Outboard motors also produce current if run in a fixed position. However, driving a boat
in the pond with an outboard in order to stir the water
is practically useless.
5. Water Replacement. Water replacement is also very
effective. Unfortunately, most pond owners do not
have an alternative water source with enough volume
to be effective.
6
1. Add six to eight pounds of potassium permanganate
per acre-foot. This oxidizes organic material in the
water and reduces the demand for oxygen. The water
will become a purple color. If the color disappears
within one hour, repeat treatment at one-half the rate
This material can be toxic to fish, so avoid over-treatment.
2. Add 50 to 100 pounds of triple sunerphosphate per
surface acre. This increases the productlor .?f oxvgep
by plants.
3. Add 50 pounds of builders or hydrated ‘ime per SI.~’
face acre This reduces carbon dioxide 13 ?he w%=
and makes the remaining oxygen more avallabir- :i
fish. This material can be toxic to fish so a\~?13 o\:r’*
treatment.
Any one of the chemical treatments car? be useo
alone, but the most effective way IS to use ai! three Usp
potassium permanganate in the rnornrng as soon 2s
signs of a depletion are observed. Treat w!tr-! t%ie
superphosphate during midday and fo!!ow WI:? builders
or hydrated lime at dusk I? is advisable to pL,rcha?:e the
required amounts of these chemicals before oxygen df
pletion problems are detected, Once oxygen depletio:‘.
is a problem, immediate treatment is essential !r some
areas these chemicals are not readily available and a
fish kill can happen in a matter of hours. Storing rbese
chemicals near the pond will save time and help prevent
a fish kill.
The key to preventing oxygen depletion IS proper
pond management. After the emergency has passed.
find and eliminate the cause of oxygen depletion. Oxygen depletion can weaken and stress surviving fish
making them more susceptible to diseases and parasites. Surviving fish should be watched closely during
the few weeks following oxygen depletion to determine
if treatment is necessary.
If a high percentage of the fish have died as a result
of oxygen depletion, it may be necessary to drain and
restock the pond. If there is any question about the severity of a kill, have the population checked by a Department of Natural Resources biologist the following summer. Biologists can be contacted through your County
Extension Agent or Conservation Ranger
7
Supplemental Aeration
There are a number of aerators on the market designed to provide supplemental aeration to a pond. Supplemental and emergency aeration methods should not
be confused. Supplemental methods are used to prevent oxygen depletion from occurring while emergency
methods are used to correct oxygen depletion once it
has occurred. Although emergency methods are effective, they are usually too difficult and expensive to use
continuously. Emergency methods include adding
chemicals to a pond such as potassiurn permanganate
and hydrated lime, pumping or agitating large volumes
of water, or flushing the pond with aerated water
Most supplemental aerators pump or agitate water to
oxygenate it or inject air directly into the water Supalemental aerators can be used in emergency situatrons
but most of them do not reoxygenate water as rapidly
as emergency aerators
For a commercial fish farmer, the use of supplemental aeration can improve water quality and production
and decrease losses. This is especially true if adequate
amounts of water are not available to periodically flush
!he pond during the hot summer months. For the
sportfishing pond owner, the positive benefits of supplemental aeration are more questionable. Most oxygen
depletions in sportfishing ponds are the result of a management error by the owner. Usually oxygen depletion
in a sportfishing pond is the result of overstocking, overfeeding, over-fertilization, chemical treatment of aquatic
weeds or pollution from barns and feedlots. These management problems coupled with hot summer weather,
summer storms, cloudy weather or strong winds can
cause oxygen depletion. In the long run, it is probably
more economical for the sportfishing pond owner to correct the management problem rather than compensate
for the problem with supplemental aeration.
The Cooperative Extension Service, The University Of Georgia
College of Agricultural and Environmental Sciences offers
educational programs, assistance and materials to all people
without regard to race, color, national origin, age, sex or handicap
status.
AN EQUAL OPPORTUNITY EMPLOYER
Leaflet 233
F&h2
Reprinted March, 1993
Issued in furtherance of Cooperative Extension work, Acts of May
8 and June 30, 1914, The University of Georgia College of
Agricultural and Environmental Sciences and the US Department
of Agriculture cooperating.
C. Wayne Jordan, Director
J
93-91